What the student, Wilken-Jon von Appen, received in return was a letter that not only turned him down but added an ominous warning from John M. Busch, a security administration official: “I have determined that you pose a security threat.”

Similar letters have gone to 5,000 applicants across the country who have at least initially been turned down for a Transportation Worker Identification Credential, an ID card meant to guard against acts of terrorism, agency officials said Monday.

During the Paleocene-Eocene Thermal Maximum more than 55 million years ago, insects chewed large holes in this leaf.

More than 55 million years ago, the Earth experienced a rapid jump in global carbon dioxide levels that raised temperatures across the planet. Now, researchers studying plants from that time have found that the rising temperatures may have boosted the foraging of insects. As modern temperatures continue to rise, the researchers believe the planet could see increasing crop damage and forest devastation.